As a broker I have had many the phone calls from friends saying they sold the boat on there own and they didnt need me as a broker. Many end in the "I carried the note and the buyer isnt paying.. how do I get it back"? Next call... "Its ruined... Im gonna file against the guy"... Next call..."Can you help me find him"?
I have never recieved the "I carried the note for many years and everything went great... we are going for drinks tonight!"

...Neither a borrower, nor a lender be;
For loan oft loses both itself and friend,
And borrowing dulls the edge of husbandry.

--William Shakespeare

Now, if you insist on doing it, and I have, over collateralize. Take a mortgage on everything you can to secure the loan, and make sure you get mortgages on property from both owners. Real estate is best. That way both have a bunch to lose and you will be the last person that gets screwed if one goes belly up -- the other guy has a huge interest in making sure you are paid.

Make sure they are both liable for the full amount of the loan, not just their portion. Then make sure you record your security interest to the extent your state allows it. Finally, include an insurance requirement in case something happens to the boat.

If they think you are being greedy by over-collateralizing, tell them it won't matter because they can obviously afford it or they wouldn't be asking a friend to tote the note. How much they give as collateral won't matter since they are going to pay.